The field of the invention is specific applications of photo curable pre-ceramic polymer chemistry to specific applications.
Commercially available high temperature ceramic matrix composites are limited to carbon fiber/carbon matrix, carbon fiber/SiC matrix, SiC fiber/SiC matrix, and more recently, carbon or SiC fiber in a silicon nitride/carbide matrix. The upper use temperature is limited to below 1600 degrees centigrade at best for all but carbon/carbon, which is highly susceptible to oxidation above 400 degrees centigrade. Carbon/carbon can be utilized at ultra high temperatures (above 2000 degrees centigrade) but only in a non-oxidizing environment. The limitations of carbon/carbon, the only truly ultra high temperature CMC system currently available, and the need for new ceramic materials was summarized by Opeka quite recently: “Ultrahigh temperature applications such as combustion chamber liners, rocket thrusters, thermal protection systems for carbon-carbon composites, and leading edges of the spacecraft require materials, which are protective and oxidation resistant at temperatures higher than 2000 degrees centigrade. Refractory ceramics such as hafnium diboride (HfB2), hafnium carbide (HfC) and hafnium nitride (HfN) are candidate materials because of their high melting points, low coefficient of thermal expansion, high erosion and oxidation resistance.” Arvind Agarwal, Tim McKeechnie, Stuart Starett and Mark M. Opeka, Proceedings for the symposium of Elevated Temperature Coatings IV. 2001 TMS Annual Meeting New Orleans, La., pp. 301-315.
U.S. Pat. No. 5,332,701 teaches ceramic compositions that can be formed by the pyrolysis of a particulate metal. The particulate metal forms a component of the ceramic and another metal that forms another component of the ceramic.
The rational for producing a nanocomposite, rather than phase pure HfC or HfN, is that the presence of both carbon and nitrogen hinder the formation of long-range order and allow the HfCN nanocomposite to be processed at high temperature in an amorphous “glassy” state prior to crystallization. This retention of the “glassy” state to high temperatures (>1400 degrees centigrade) in the silicon nitride/carbide (SiNC) system has been seen. In the case of HfCN, the temperature of crystallization should be even higher due to the fact that hafnium is tetravalent in HfC and trivalent in HfN. In addition, the melting points of HfC and HfN are significantly higher than that of silicon carbide and silicon nitride.
Numerous pre-ceramic polymers with improved yields of the ceramic have been described in U.S. Pat. No. 5,138,080, U.S. Pat. No. 5,091,271, U.S. Pat. No. 5,051,215 and U.S. Pat. No. 5,707,471. The fundamental chemistry contained in these embodiments is specific to the process employed and mainly leaves the pre-ceramic polymer in a thermoplastic state. These pre-ceramic polymers which catalytic or photo-induced cross-linking do not satisfy the high ceramic yield, purity and fluidity in combination with low temperature cross-linking ability necessary for producing large densified ceramic structures in a single step continuous process.
U.S. Pat. No. 5,138,080 teaches a novel polysila-methylenosilane polymers which has polysilane-poly-carbosilane skeleton which can be prepared in one-step reaction from mixtures of chlorosilaalkanes and organochloro silanes with alkali metals in one of appropriate solvents or in combination of solvents thereof. Such polysilamethyleno silane polymers are soluble and thermoplastic Later versions of this polymer Me(H)SiCl2 in addition to the Me 2SiCI2 and are subjected to a sodium-hydrocarbon dechlorination process which does not attack vinyl groups. The resulting polymer consists of a predominately linear, Si—Si “backbone” bearing pendant methyl groups, with some Si—H and Si—CH≡CH2 functionality to allow crosslinking on pyrolysis.
None of these precursors derived using vinylchlorosilanes are similar to those of the process in that having predominantly Si—Si bonded “backbones”, they are essentially polysilanes, not polycarbosilanes. In addition, the carbon in these polymers is primarily in the form of pendant methyl functionality and is present in considerable excess of the desirable 1 to 1 ratio with silicon. The ceramic products obtained from these polymers are known to contain considerable amounts of excess carbon.
Polymeric precursors to SiC have been obtained by redistribution reactions of methyl-chloro-disilane (Me6-xClxSi2, x=24) mixtures, catalyzed by tetraalkyl-phosphonium halides which U.S. Pat. No. 4,310,481, U.S. Pat. No. 4,310,482 and U.S. Pat. No. 4,472,591 teach. In a typical preparation, elemental analysis of the polymer was employed to suggest the approximate formula [Si(Me)1.15(H)0.25]n, with n averaging about 20. The structures of the polymers involve what is reported to be a complex arrangement of fused polysilane rings with methyl substitution and a polysilane backbone.
The formation of carbosilane polymers with pendent methyl groups has been prepared as by-products of the “reverse-Grignard” reaction of chloromethyl-dichloro-m-ethylsilane. The chief purpose of this work was the preparation of carbosilane rings and the polymeric byproduct was not characterized in detail nor was its use as a SiC precursor suggested. Studies of this material indicate that it has an unacceptably low ceramic yield on pyrolysis. These polymers contain twice the required amount carbon necessary for stoichiometric silicon carbide and their use as SiC precursors was not suggested. Moreover, the starting material, chloromethyl-dichloro-methylsilane, contains only two sites on the Si and can be pyrolyzed to obtain improved yields of silicon carbide at atmospheric pressure.
U.S. Pat. No. 5,051,215 teaches a rapid method of infusibilizing pre-ceramic polymers that includes treatment of the polymers with gaseous nitrogen dioxide. The infusibilized polymers may be pyrolyzed to temperatures in excess of about 800.degree. C. to yield ceramic materials with low oxygen content and, thus, good thermal stability. The methods are especially useful for the production of ceramic fibers and, more specifically, to the on-line production of ceramic fibers.
U.S. Pat. No. 4,847,027 teaches a method for the preparation of ceramic materials or articles by the pyrolysis of pre-ceramic polymers wherein the pre-ceramic polymers are rendered infusible prior to pyrolysis by exposure to gaseous nitric oxide. Ceramic materials with low oxygen content, excellent physical properties, and good thermal stability can be obtained by the practice of this process. This method is especially suited for the preparation of ceramic fibers.
U.S. Pat. No. 4,631,179 teaches a ring-opening-polymerization reactions method to obtain a linear polymer of the formula [SiH2CH2]n. This polymer exhibit ceramics yields up to 85% on pyrolysis. The starting material for the ring-opening-polymerization reaction was the cyclic compound [Si—H2CH2]2, which is difficult and costly to obtain in pure form by either of the procedures that have been reported.
U.S. Pat. No. 5,153,295 teaches compositions of matter that have potential utility as precursors to silicon carbide. These compositions are obtained by a Grignard coupling process. The process starts from chlorocarbosilanes and a readily available class of compounds. These polymers have the advantage that it is only necessary to lose hydrogen during pyrolysis, thus ceramic yields of over 90% are possible, in principle. The extensive Si—H functionality allows facile crosslinking and the 1 to 1 carbon to silicon ratio and avoids the incorporation of excess carbon in the SiC products that are ultimately formed. The synthetic procedure employed to make them allows facile modification of the polymer, such as by introduction of small amounts of pendant vinyl groups, prior to reduction. The resulting vinyl-substituted “SiH.sub.2 CH.sub.2” polymer has been found to have cross-linking properties and higher ceramic yield.
A pre-ceramic polymer has been prepared by a thermally induced methylene insertion reaction of polydimethylsilane. The resulting polymer is only approximately represented by the formula [SiHMeCH2]n, as significant amounts of unreacted (SiMe2)n units, complex rearrangements, and branching are observed. In addition to the carbosilane “units”, large amounts of Si—Si bonding remains in the “backbone” of the polymer. This polymer disadvantageously contains twice the stoichiometric amount of carbon for SiC formation. The excess carbon must be eliminated through pyrolytic processes that are by no means quantitative. Despite the shortcomings, this polymer has been employed to prepare “SiC” fiber. However, it must be treated with various crosslinking agents prior to pyrolysis which introduce contaminants. This results in a final ceramic product that contains significant amounts of excess carbon and silica which greatly degrade the high temperature performance of the fiber.
SiC precursors, predominately linear polycarbo-silanes, have been prepared via potassium dechlorination of chloro-chloromethyl-dimethylsilane. The resulting polymers have not been fully characterized, but probably contain significant numbers of Si—Si and CH2—CH2 groups in the polymer backbone. The alkali metal dechlorination process used in the synthesis of such materials does not exhibit the selective head-tail coupling found with Grignard coupling. The pendant methyl groups in such materials also lead to the incorporation of excess carbon into the system. In several polymer systems mixtures containing vinylchlorosilanes (such as CH2≡CH—Si(Me)Cl2) and Me2SiCl2 are coupled by dechlorination with potassium in tetrahydro-furan. U.S. Pat. No. 4,414,403 and U.S. Pat. No. 4,472,591 both teach this method. The “backbone” of the resulting polymers consists of a combination of Si—Si and Si—CH2CH(—Si)2 units. atom for chain growth and therefore cannot yield a structure which contains tbd.SiCH.sub.2-chain units. On this basis, the structure of the polymer obtained, as well as its physical properties and pyrolysis characteristics, is not optimal for use as an SiC precursor.
U.S. Pat. No. 4,631,179 teaches a polymer which is a product of the ring-opening polymerization of (SiH2CH2)2 also has the nominal composition “SiH2CH2”. However, the actual structure of this polymer is reported to be a linear polycarbosilane which presumably has only [SiH2CH2] as the internal chain segments. The (SiH2CH2)2 monomer used by Smith is difficult and expensive to prepare and not generally available.